FIG. 5 is a longitudinal sectional view showing the construction of a vacuum pump. The vacuum pump has a main housing 1, a suction side housing 2 attached on a right end surface of the main housing 1, a discharge side housing 3 attached on a left end surface of the main housing 1, and a gear housing 4 mounted in the left side of the discharge side housing 3. On a left end portion of the gear housing 4, a motor 5 is mounted.
The main housing 1 is provided with an inner cylinder 1a longitudinally extending therethrough, a suction port 6 externally cummunicating with the inner cylinder la at a right side of the inner cylinder 1a, and a cooling water chamber 7 for cooling a wall of the main housing 1.
The inner cylinder 1a accommodates a pair of screw rotors 8 engaging with each other (only one of them is illustrated in FIG. 5).
The suction side housing 2 is formed with two recesses in which a pair of bearing caps 9 (only one of them is illustrated in FIG. 5) are received to be secured therein. Each bearing cap 9 accommodates a bearing 10 for rotatably supporting a shaft 8a extending from a right end of the screw rotor 8.
The discharge side housing 3 is formed with two recesses in which a pair of bearing caps 11 (only one of them is illustrated in FIG. 5) are received to be secured therein. Each bearing cap 11 accommodates a bearing 12 for rotatably supporting a shaft 8a extending from a left end of the screw rotor 8.
Each screw rotor 8 has a tooth portion 8b engaging with another tooth portion 8b of the opposing screw rotor 8. One of the screw rotors 8 is a driving rotor. On an outer surface of the left side shaft 8a of the driving rotor, a timing gear 24 is secured. In a left side of the timing gear 24, there is mounted a coupling 25 which is coupled to an output shaft 5a of the motor 5.
The other screw rotor 8, which is driven by the rotation of the one of the screw rotors 8, has another timing gear (not shown) engaging with the former timing gear 24 and secured on a shaft 8a attached on a left portion of the other screw rotor 8.
The rotation of the screw rotor 8 draws in a fluid (a gas) from the suction port 6 to discharge it from a discharge port 13.
The vacuum pump generates heat during its operation to heat itself up to a high temperature. This high temperature causes a damage of an oil seal or a lip seal for axially sealing the shaft of the screw rotor 8 or of the bearing supporting each end of the screw rotor 8. The high temperature may also cause another problem such as seizing of the screw rotors 8. Therefore, a water cooling system has to be provided for the vacuum pump.
Thus, the discharge side housing 3 is provided with the discharge port 13 communicating with the inner cylinder la and a cooling water chamber 19 for cooling a wall of the discharge side housing 3.
The gear housing 4, which is cylindrical, has a cooling water chamber 14 on an outer surface thereof, and a cooling water chamber 15 is provided on an outer surface of the motor 5.
The cooling water of the vacuum pump flows, as illustrated in FIG. 6, into the cooling water chamber 15 of the motor 5 through a cooling water supply line 16 to cool the motor 5 and thereafter is delivered into the cooling water chamber 14 of the gear housing 4 through a connecting pipe 17 to cool the gear housing 4.
The cooling water which has cooled the gear housing 4 flows through a connecting pipe 18 into the cooling water chamber 19 of the discharge side housing 3 to cool the discharge side housing 3 and then is delivered into the cooling water chamber 7 of the main housing 1 through a connecting pipe 20. After the cooling water has cooled the main housing 1, the cooling water flows through a connecting pipe 21 into a cooling water chamber 22 of the suction side housing 2 to cool the suction side housing 2 and finally is discharged from a discharge line 23.
Thus, the heat generated in operation of the vacuum pump is removed.
A dry vacuum pump used in a semiconductor producing apparatus has to accomplish a vacuum degree of the order of 1 Pa (of 10.sup.-3 Torr). When a gas handled by the vacuum pump is finally discharged into the atmosphere, the gas should be compressed at a compression rate of the order of 10.sup.5 before the discharge, generating a large amount of heat due to the compression.
Therefore, a cooling system with a cooling water is inevitable for the vacuum pump as well as a general vacuum pump. However, a disadvantage of the vacuum pump remains as described in the following.
The cooling of the main housing 1 of the dry vacuum pump cools a process gas flowing in the main housing 1, so that substances such as AlCl and NH.sub.3 Cl contained in the gas changes into solids which deposit on the inner cylinder 1a or on the screw rotors 8. The deposits block a clearance between the pair of the screw rotors 8 and a clearance between the screw rotors and the inner cylinder 1a, interrupting the rotation of the screw rotors 8.
The vacuum pump has been used in various applications in semiconductor producing steps. For example, the vacuum pump is used in a light process generally called as a clean process in which no deposits are generated. The light process, in which a conventional vacuum pump may be used with no problem, is applied in a load lock process and a sputtering process. However, deposits are generated during a process of CVD (Chemical Vapor Deposition) such as Nitride or Teos for covering a thin film on a wafer. Also, deposits are generated during an Al etching process.
During the Nitride process, chemical substances react as follows. EQU SiH.sub.2 Cl+NH.sub.3.fwdarw.Si.sub.3 N.sub.4 +NH.sub.4 Cl
During the Al etching process, chemical substances react as follows. EQU Al+Cl.sub.2.fwdarw.AlCl.sub.2
That is, the solid of NH.sub.4 Cl or AlCl.sub.2 is produced.
NH.sub.4 Cl sublimes to become a gas from a solid at a temperature more than 180.degree. C. under a normal atmospheric pressure. NH.sub.3 Cl sublimes at a temperature of around 338.degree. C.
In a vacuum state in which only an attenuated gas is existing, no deposits are generated. Thus, a method, in which N.sub.2 is purged into a discharge side of the screw rotor, has been proposed to prevent the generation of deposits in a compression stage. However, the method is still insufficient.
Furthermore, in the semiconductor producing process including a light process and a hard process, it is disadvantageous for management of the producing process in that the two types of vacuum pumps have to be prepared for an alternate use thereof.
The present invention can be applied to a method including a N.sub.2 purge step and a heating step. However, in the heating step, a conventional electric heater is not used, but the deposit generation is limited by controlling heat generated by compression during operation of a vacuum pump. Furthermore, the present invention provides a dry vacuum pump which is advantageously used for a light process and also for a heavy process with a one-touch switching operation.